The invention relates to wavelength selective switches, and relates in particular to wavelength selective switches that may be used in a multiplexing switching system.
Wavelength selective switches include a variety of structures such as fiber grating couplers that permit all but a specified wavelength of light to pass through the coupler. For example, U.S. Pat. No. 4,737,007 discloses a wavelength selective contra-directional coupler for use in wavelength division multiplexing that includes a Bragg diffraction grating having a fixed period. U.S. Pat. No. 6,549,707 discloses a grating type optical filter that provides an apodized response by varying the distance between grating elements.
Superimposed multiple Bragg gratings are disclosed in A. Othonos, X. Lee and R. M. Measures, “Superimposed Multiple Bragg Gratings”, Electronics Letters, Vol. 30, No. 23, pp. 1972-1974, (1994) in which more than one grating is inscribed at the same location on an optical fiber to provide a comb filter by varying the period of different grating elements. Serial inscription of a second grating, however, typically results in a reduction of reflectance of an first grating as disclosed in W. C. Swann, M. A. Hubbard and S. L. Gilbert, “Hybrid Multiple Wavelength Reference Using Fiber Grating and Molecular Absorption”, Proc. Bragg Gratings, Photosensitivity and Poling in Glass Wavelength, OSA Technical Digest (Optical Society of America), pp. 63-65 (1999). It has also been disclosed that performance may be degraded with an increase in the number of superimposed gratings in H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous Wavelength Tuning in Super-Structure-Gratings (SSG) DBR Lasers”, IEEE Journal of Quantum Electronics, Vol. 32, No. 3, pp. 433-441 (1996).
Multi-wavelength diffraction and apodization are disclosed using binary superimposed gratings in I. A. Avrutsky, M. Fay and J. H. Xu “Multiwavelength Diffraction and Apodizatrion Using Binary Superimposed Gratings”, IEEE Photonics Tech. Letters, Vol. 10, No. 6, pp. 839-841, (1998) in which the grating is segmented into sections of grouped segments permitting step-like apodization while maintaining a binary index and base segment size.
A sampled grating is disclosed in V. Jayaraman, Z. Chuang, and K. Coldren, “Theory, Design, and Performance of Extended Tuning Range Semiconductor Lasers With Sampled Gratings”, IEEE Journal of Quantum Electronics, Vol. 29, No. 6, pp. 1824-1834 (1993) in which grating elements are removed in a periodic fashion.
There is a need however, for a more efficient and economical wavelength selective switch that may be used in a multiplexing switching system, and that may be modified during use.